Hyperpolarization of a gas

ABSTRACT

The present invention relates to a method of hyperpolarizing a gas sample. The method cryogenically forming a solidified gas structure from the sample gas, the solidified gas structure being surrounded by  3 He. A magnetic field is then to the solidified gas structure and the  3 He to thereby polarize the solidified gas structure, before the  3 He is removed to thereby leave a solidified gas structure of hyperpolarized sample gas.

[0001] The present invention relates to a method and apparatus forhyperpolarizing a gas sample. In particular, the present inventionrelates to hyperpolarizing a noble gas for use in MRI and NMRexperiments.

[0002] Conventional MRI techniques exploit the interaction of theintrinsic magnetic moment or spin of nuclei with an applied magneticfield. Nuclei whose spin is aligned with the applied magnetic field havea different energy state to nuclei whose spin is aligned opposed to theapplied magnetic field. Accordingly, by applying a radio frequencyradiation to the nuclei in a magnetic field, nuclei can be made to jumpfrom a lower energy state to a higher energy state. The signals producedwhen the nuclei return to the lower energy state can then be measured,thereby providing information concerning the nature of the physicalproperties of the object being measured.

[0003] In most cases, MRI and NMR imaging is carried out using hydrogennuclei which are present in water and fat. However, suitable nuclei arenot always naturally present to enable measurements to be made. Thus,for example, in the lungs there are too few protons to generate a clearimage. In addition to this microscopic air/tissue interfaces of the lungproduce magnetic field variations that cause the already weak signal todecay even more rapidly. The problem is further exasperated by normalbreathing and cardial motion.

[0004] A proposed solution to this is for the patient to inhale amixture of a buffer gas, such as Nitrogen or Helium, and a stronglypolarised sample gas, such as a noble gas. The hyperpolarized noble gas,as it is known, is a gas which includes an induced polarization, andhence an induced magnetic moment in the atomic nuclei. This allows MRIand NMR experiments to be performed in the normal way, even if thenormally used hydrogen nuclei are not present.

[0005] Currently there are two main techniques for generatinghyperpolarized gases. The first technique is described for example inU.S. Pat. No. 5,809,801, U.S. Pat. No. 5,617,860 and U.S. Pat. No.5,642,625.

[0006] The technique described in these documents is the indirecthyperpolarization of Xenon or Helium3 (³He). This is achieved by mixingthe gas with a small amount of an alkaline-metal vapour, such asrubidium. A weak magnetic field is applied to the vapour mixture tocause splitting of the alkaline-metal electron energy levels.

[0007] The vapour mixture is then optically pumped using a laser tocause a build-up of electron polarization in the higher energy sub-levelof the metal vapour. Nuclei of the noble gas atoms then become polarizedby collisions with the alkaline-metal which causes transfer of angularmomentum from the polarized alkali electrons to the nuclei spin of thenoble gas.

[0008] An alternative method of hyperpolarizing ³He is achieved bydirect optical pumping of a metastable state of the helium. In thismethod an electrical discharge and a low pressure cell are used tocreate atoms of ³He in a metastable state. These metastable atoms arethen exposed to circularly polarized laser light, from a high poweredLNA-laser, which causes the transfer of polarization from the electronsto the helium nuclei via coupling with the unpaired neutron.

[0009] Both of the above mentioned methods rely on optical pumpingtechniques and are therefore extremely inefficient.

[0010] Other methods have been considered which involve the use ofsolidified Xenon. However, the Xenon has a long spin-lattice relaxationtime and therefore must be kept in a strong magnetic field, at a lowtemperature, for long periods of time to result in any useful level ofpolarization. This direct approach is therefore impractical.

[0011] In order to overcome this, the document “High EquilibriumSpin-Polarizations in Solid ¹²⁹Xenon” by Honig et al, proposes mixingthe Xenon with bulk amounts of oxygen to help improve the polarization.Meanwhile “The Brute Force ¹²⁹Xe and D₂ Polarization at low temperature”by Usenko et al describes achieving polarization by inducing an electroncurrent in the solidified Xenon. Again however, these techniques haveproved to be extremely inefficient.

[0012] FROSSATI G: “Polarisation of He, D2 (and possibly Xe) usingcryogenic techniques” NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH,SECTION—A: ACCELERATORS, SPECTROMETERS, DETECTORS AND ASSOCIATEDEQUIPMENT, NORTH-HOLLAND PUBLISHING COMPANY .AMSTERDAM, NL, vol. 402,no. 2-3, Jan. 11, 1998, pages 479-483 discloses a method ofhyperpolarizing a gas sample, the method comprising the steps ofcryogenically forming a solidified gas structure from the sample gas,the solidified gas structure being surrounded by 3He, applying amagnetic field to the solidified gas structure and the 3He to therebypolarize the solidified gas structure, and removing the 3He to therebyleave a solidified gas structure of hyperpolarized sample gas.

[0013] In accordance with a first aspect of the present invention, weprovide a method of hyperpolarizing a gas sample, the method comprisingthe steps of:

[0014] a. cryogenically forming a solidified gas structure from thesample gas, the solidified gas structure being surrounded by ³He;

[0015] b. applying a magnetic field to the solidified gas structure andthe ³He to thereby polarize the solidified gas structure; and

[0016] c. removing the ³He to thereby leave a solidified gas structureof hyperpolarized sample gas, characterized in that step (c) comprisesthe steps of:

[0017] i. increasing the temperature of the solidified gas structure;

[0018] ii. introducing ⁴He into the region surrounding the solidifiedgas structure to thereby displace the ³He; and

[0019] iii. pumping the ³He and the ⁴He away from the solidified gasstructure.

[0020] In accordance with a second aspect of the present invention, weprovide apparatus for hyperpolarizing a gas sample, the apparatuscomprising:

[0021] a. cryogenic apparatus for forming a solidified gas structurefrom the sample gas, the solidified gas structure being surrounded by³He;

[0022] b. magnetic field generating assembly for applying a magneticfield to the solidified gas structure and the ³He to thereby polarizethe ³He and the solidified gas structure; and

[0023] c. a removal system for removing the ³He to thereby leave asolidified gas structure of hyperpolarized noble gas, characterized inthat the removal system is adapted to carry out the steps of:

[0024] i. increasing the temperature of the solidified gas structure;

[0025] ii. introducing ⁴He into the region surrounding the solidifiedgas structure to thereby displace the ³He; and

[0026] iii. pumping the ³He and the ⁴He away from the solidified gasstructure.

[0027] Accordingly, the present invention provides a method and anapparatus for producing a hyperpolarized sample gas. In this technique,a solidified gas structure is formed which is surrounded by ³He. Amagnetic field is then applied to the gas structure and the ³He whichcauses polarization of the solidified gas structure. Under conditions oflow temperature, solidified gases normally have an extremely lowrelaxation rate. However, in the present invention magneticdipole-dipole coupling at the gas structure/³He interface leads to anincrease in the relaxation rate of the solidified gas, therebyincreasing the rate at which the solidified gas is polarized. The ³He isthen removed to leave behind a solidified gas structure ofhyperpolarized noble gas. This can then be used in NMR and MRIexperiments as required.

[0028] The step of cryogenically forming a solidified gas structuresurrounded by ³He usually comprises the steps of forming a solidifiedgas structure and, introducing the ³He into the regions surrounding thesolidified gas structure. Alternatively however the solidified gasstructure is formed in an environment including ³He.

[0029] Typically the step of forming a solidified gas structurecomprises the step of cooling a substrate within a chamber and,introducing the sample gas into the chamber thereby causing the samplegas to condense onto the substrate. However, alternatively the gasstructure may be formed by introducing the sample gas into anenvironment including a substrate and then cooling the entireenvironment to cause the sample gas to condense onto the substrate.

[0030] Typically the introduction of the sample gas into the chamber iscontrolled such that the sample gas condenses to form a solidified gaslayer on the substrate, the layer having a thickness of about 10monolayers. This is particularly advantageous as the increasedrelaxation rate induced by the ³He is only effective for the uppermostlayers of the solidified sample gas structure. Layers of the solidifiedgas further from the ³He will become polarized by spin diffusion effectswhich take time to propagate through the sample. For layers polarized bythis technique, the effect of the ³He on the relaxation rate is reducedand it is therefore preferable to ensure that as much of the solidifiedgas structure as possible is in contact with the ³He to thereby ensureas high a relaxation rate as possible. However, thicker layers of noblegas may be used if the ³He is maintained in contact with the noble gasfor longer periods of time.

[0031] It is preferable that the substrate is a porous medium so as toensure the formation of a gas structure with a high surface area.Furthermore, it is preferable to ensure a low concentration ofparamagnetic contaminants. Accordingly, the substrate is typically afumed silica. However, any suitable substance having a high surface areacould be used, such as activated carbon, exfoliated graphite, clays,porous glasses, zeolites, silica areogels, and silicas may also be used.

[0032] The solidified gas structure is preferably porous and has a largesurface area. This ensures that as much of the noble gas as possible canbe condensed and then polarized whilst interacting with the ³He therebyensuring the polarization times are kept to a minimum. However, areduced surface area could be used if other parameters such as thetemperature of the substrate and the applied magnetic field are adjustedaccordingly.

[0033] Typically the step of introducing the ³He into the regionsurrounding the solidified gas structure comprises precooling thesolidified gas structure and, immersing the precooled solidified gasstructure in liquid ³He. In this case, because the gas structure has alow heat capacity, it can be cooled to a temperature of below 4.2K andthen immersed in the liquid ³He without causing an undue temperaturerise in the ³He, which is itself typically cooled to a temperature ofless than 100 mK and preferably to a temperature of 10 mK. Thistechnique also ensures the entire surface of the solidified gasstructure will be in contact with the ³He to ensure adequate bondingoccurs. Furthermore, this allows the ³He to be maintained at a lowtemperature (below 100 mK) thereby overcoming the need to repeatedlycool the ³He.

[0034] However, the introduction of ³He into the area surrounding thegas structure may alternatively involve the steps of introducing the ³Heinto a chamber containing the solidified gas structure so as to fill thepores of the solidified gas structure and cooling the solidified gasstructure to cause a ³He to solidify on the surface of the solidifiedgas structure. A further alternative is for the solidified gas structuremay be precooled so that the ³He condenses and solidifies as soon as ithas entered the chamber.

[0035] The method preferably further comprises cooling the solidifiedgas structure and the ³He to a temperature of less than 100 mK. In thiscase, the magnetic field applied to the solidified gas structure and the³He has a strength of between 10 and 20T. However, alternative values oftemperature and magnetic field can be used by adjusting other parametersto compensate for any reduction in the induced polarization. Thus forexample, if a reduced magnetic field is used, the magnetic field couldbe applied for a longer periods of time. Ultimately however, the levelof polarization will depend on the applied field and the inverse oftemperature. It is therefore preferable to use as large a field aspossible at a low temperature.

[0036] Typically the sample gas is a noble gas sample which comprises¹²⁹Xe. However, any suitable sample gas such as ¹³¹Xe, ⁸³Kr ³⁹Ar, ²¹Ne,¹⁵N, ²D or the like, could be used. Accordingly, this does not representan exhaustive list of the samples which can be polarised using thetechnique of the present invention.

[0037] Alternatively however, the sample gas could comprise ³He in whichcase, the ³He is preferably formed as a layer of solid on a substrate,with the solidified gas structure being surrounded by liquid ³He. Thistherefore allows hyperpolarized helium to be produced by the techniquesof the present invention.

[0038] The method preferably further comprises the step of storing thehyperpolarized gas in the form of the solidified gas structure bymaintaining the solidified gas structure in a magnetic field having astrength of between 50 mT and 1T at a temperature of below 10K. However,the gas may alternatively be vaporised and used immediately. Thehyperpolarized gas can be obtained from the solidified gas structure byraising the temperature of the solidified gas structure to above 161K.

[0039] As previously described, the hyperpolarized gas is then suitablefor use in NMR and MRI type analysis techniques. In the case of MRI, thehyperpolarized gas can be mixed with a buffer gas and then inhaled,allowing details of the lungs to be determined.

[0040] The cryogenic apparatus usually comprises a cell containing aporous substrate, the cell having an input for receiving the noble gas;a cooling system; and, a controller for controlling the cooling system,the controller being adapted to cause the cooling system to cool thecell thereby causing the noble gas to condense on the substrate so as toform a porous solidified gas structure.

[0041] Typically the removal system comprises a pump coupled to thecell, the controller operating to control the cooling system to raisethe temperature of the cell so as to cause evaporation of the ³He, thepump being adapted to remove the ³He vapour from the cell. However, thecontroller could be adapted to raise the temperature of the cell tovaporise both the noble gas and the ³He. In this case, the apparatuswould further comprise means for separating the resulting gas mixture.

[0042] Preferably the removal system further comprises a source of ⁴He,the source being coupled to the input to supply ⁴He into the cell, the⁴He causing displacement of the ³He from the solidified gas structure.However, the removal of the ³He can be implemented without the additionof the ⁴He.

[0043] Typically the controller is also coupled to the magnetic fieldgenerating apparatus for controlling the magnetic field applied to thecell. This allows the applied field and the temperature of thesolidified gas structure to be accurately controlled by a singleelement, thus ensuring that the solidified gas structure is subject tothe ideal conditions for polarization.

[0044] The magnetic field generating apparatus will usually comprise atleast one superconducting coil. However, several separate sets of coilsmay be used to ensure that fields of the desired strength andhomogeneity can be generated for the entire solidified gas structure. Asa result, the magnetic filed generating apparatus may also includepermanent magnets.

[0045] An example of the present invention will now be described withreference to the accompanying drawing, which:

[0046]FIG. 1 is a schematic diagram of apparatus for generating ahyperpolarized noble gas in accordance with the present invention.

[0047]FIG. 1 shows a system suitable for generating a hyperpolarized gasin accordance with the present invention.

[0048] The system includes a cell 1 which is coupled to the cold end ofa top loading refrigerator probe 2 via an attachment member 3. Theentirety of the refrigerator probe system is not shown in the currentdrawing for clarity purposes although a pressed silver sinter seal 6 isshown which is used for coupling the probe 2 to the remainder of therefrigeration system.

[0049] The cell 1 is coupled to the attachment member 3 via a first seal4 and the attachment member 3 is in turn coupled to the probe 2 via asecond seal 5 which is typically formed from indium or the like.

[0050] The cell would typically include some pressed silver sinter toensure good thermal contact with the probe 2.

[0051] Positioned at the open end of the cell 1 is a supply line 7 whichis capable of supplying gases and liquids into the cell 1. A vacuum pump(not shown) is also provided for removing gases from inside the cell 1.

[0052] Finally, positioned around the cell 1 are a number of NMR coils 9and a high-field magnetic generating assembly 10. These are controlledby separate driving circuits (not shown) as will be described in moredetail below.

[0053] In this example, it will be noted that the cell 1 is a long thincylinder. The use of a long thin cylinder is particularly advantageousas it will tend to minimise demagnetisation filled gradients which wouldarise if the cell included sharp corners, such as if it were rectangularin shape.

[0054] The cell typically has a volume in the region of 1 cm³ althoughit will be appreciated that the apparatus can be scaled up to largervolume cells. With a cell of this size, in this example, the porousmedium 8 is formed from approximately 0.1 gram of fumed silica which hasan approximate surface area of 40 m².

[0055] The basic procedure for polarizing a noble gas, which in thisexample is ¹²⁹Xe involves the following steps:

[0056] 1. Coat the porous medium with ¹²⁹Xe;

[0057] 2. Coat the ¹²⁹Xe with ³He;

[0058] 3. Polarize the ¹²⁹Xe;

[0059] 4. Remove the ³He; and,

[0060] 5. Store the hyperpolarized ¹²⁹Xe.

[0061] Each of these procedures will now be explained in more detailbelow.

[0062] Coating the Porous Medium

[0063] Once the porous medium 8 is positioned in the cell 1, the cell 1is pumped down to a vacuum using the vacuum pump and cooled. Once thishas been completed, the ¹²⁹Xe gas is introduced via the supply line 7.

[0064] As the ¹²⁹Xe is introduced into the cell 1, the reducedtemperature of the porous medium 8 causes the ¹²⁹Xe to condense. Thismay be achieved under a variety of conditions.

[0065] A first example is to cool the cell 1 such that the porous medium8 is held at a temperature of well below 100 Kelvin. The ¹²⁹Xe gas isthen admitted in a slow controlled manner so that ¹²⁹Xe condensesdirectly onto the surface of the porous medium. At these temperaturesthe vapour pressure of the ¹²⁹Xe is very small, and most atoms wouldremain where they first condense.

[0066] Another possible method is to hold the porous medium at atemperature between 100 Kelvin and the triple point 161 Kelvin as the¹²⁹Xe gas is admitted. At these temperatures the ¹²⁹Xe condenses into asolid with an appreciable vapour pressure of up to 0.82 bar at thetriple point. The presence of ¹²⁹Xe vapour within the cell 1 allows fortransport of ¹²⁹Xe from one part of the porous medium to another,ideally ending up with a uniform film.

[0067] Thirdly, the porous medium may be maintained at a temperatureabove the triple point 161 Kelvin so that the ¹²⁹Xe would initiallycondense into a liquid. This liquid would flow over the surface of theporous medium thereby ensuring even coating of the surface with ¹²⁹Xe.

[0068] Alternatively, the ¹²⁹Xe gas may be introduced into the cell 1and then the cell 1 cooled to cause condensation of the ¹²⁹Xe gas ontothe surface of the porous medium.

[0069] Whichever conditions are used, the supply of the ¹²⁹Xe iscontrolled so that the porous medium 8 is coated with a layer ofapproximately 10 monolayers thickness of ¹²⁹Xe, i.e. approximately 40 Åthick.

[0070] Coating the ¹²⁹Xe

[0071] Once the formation of the ¹²⁹Xe coating has been achieved, the¹²⁹Xe coating must itself be coated with ³He. In this example, this isachieved by inserting ³He into the cell 1 through the supply line 7.This may be carried out under a variety of conditions.

[0072] Firstly, this can be performed at the temperature at which the¹²⁹Xe coating was condensed onto the porous medium, in which case the³He is input as a gas. If this occurs, the cell 1 is then cooled tocause the ³He to liquify onto the ¹²⁹Xe coating.

[0073] However, cooling the ³He to liquify it takes a long time andrequires a large amount of energy. It is therefore preferable tointroduce the ³He as a liquid. In order to achieve this the cell ispre-cooled to a temperature below 2.2K and preferably below 300 mK toensure that the ³He remains in a liquid state. In this case, as theliquid ³He typically has a temperature of 10 mK, the introduction of the³He advantageously causes further cooling of the cell 1, therebyensuring the liquid ³He does not evaporate.

[0074] The liquid helium flows over the entire surface area of the ¹²⁹Xeand binds to the ¹²⁹Xe using the van der Waals interaction between the³He and the solid molecules of ¹²⁹Xe. This binding effectivelypressurizes the first few layers of the ³He to above the solidificationpressure which is 34 bar for ³He. This causes the formation of a solidlayer of ³He on the surface of the ¹²⁹Xe.

[0075] Due to the exceptionally small atomic polarizability of the ³He,the van der Waals forces are stronger between ³He and ¹²⁹Xe atoms thanbetween two ³He atoms. This not only makes the solidification of the ³Hepossible but also has consequences for eventual removal of the ³He, aswill be explained in more detail below.

[0076] It will be realised that as an alternative, the ¹²⁹Xe coatingcould be formed on the porous medium 8 which is then dipped into a“pool” of liquified ³He to ensure total coating of the ¹²⁹Xe. In thiscase, the ¹²⁹Xe would be precooled to a temperature of below 4K and thecell 1 could be removed from the probe 2. The cell can then be dipped inliquid ³He so as to coat the ¹²⁹Xe before the cell 1 is replaced on theprobe 2. In this case, with the liquid ³He being held at a temperatureof below 300 mK, this would have the advantage that the ¹²⁹Xe is furthercooled by its immersion in the liquid ³He.

[0077] Polarization of the ¹²⁹Xe

[0078] In most substances, the primary mechanism for nuclei magneticrelaxation is the modulation of the intermolecular dipole-dipoleinteraction by atomic or motion forces. Generally, this motion is nearlycompletely frozen out at dilution refrigerator temperatures leading to avery long relaxation time. However, in solid ³He, quantum tunnellingmotion of the atoms at MHz frequencies leads to short relaxation times.

[0079] Accordingly, application of a magnetic field causes therelaxation of the ³He atoms in the solidified ³He layer, which in turncauses relaxation of the ¹²⁹Xe atoms in the solidified ¹²⁹Xe. However,due to the inverse cubed falloff of the dipole-dipole interaction withdistance, it is only the surface layers of the solidified ¹²⁹Xe whichare effectively relaxed by the dipole-dipole interaction with the ³Heatoms.

[0080] As will be appreciated by a person skilled in the art, the amountof relaxation in the ¹²⁹Xe depends on the surface area of the ¹²⁹Xe andthe relative distance between the ³He and the ¹²⁹Xe surface, as well ason the temperature, the applied magnetic field, and the duration forwhich the sample is held under the polarizing conditions.

[0081] Accordingly, providing the solidified ¹²⁹Xe with a large surfacearea, and overlaying the ³He helps maximise the polarization.

[0082] Further improvement is also obtained by ensuring the cell 1 iscooled to about 10 mK. Once this has been completed, the ³He and the¹²⁹Xe exposed to a magnetic field having a strength in the region of 16Twhich is generated using the high strength magnet arrangement 10.

[0083] Under these conditions, suitable polarisation is achieved on atime scale of about 3000 s. However, it will be realised that thesevalues can be varied significantly. For example, the longer the timeused, the more the ¹²⁹Xe becomes polarized. Thus, a reduced field couldbe used by maintaining the conditions for a longer period of time.

[0084] The level of polarisation of the sample is measured by the NMRcoils 9, allowing the process to be stopped when the sample issufficiently polarized.

[0085] Removal of the ³He

[0086] Once the ¹²⁹Xe has been polarized using the above mentionedmethod, it is then necessary to remove the ³He. This is because, as thelow temperature and high magnetic field conditions are removed, the ³Hewill rapidly relax and therefore lose polarization. This in turn wouldcause the polarization of the ¹²⁹Xe to be reduced.

[0087] The most simple technique for removing the ³He is to heat thecell 1 to above 300 mK to cause the ³He to evaporate. This then allowsthe ³He to be removed from the cell 1 using the vacuum pump.

[0088] However, this technique suffers from the drawback that thesolidified ³He layer that forms on the surface of the ¹²⁹Xe will notevaporate at such low temperatures, due to the van der Waals interactionwith the ¹²⁹Xe atoms. Accordingly, to remove the solidified ³He it isnecessary to further raise the temperature to about 4.2K to cause totalevaporation of the ³He. By the time these temperatures have beenattained and the ³He has evaporated, the ¹²⁹Xe and ³He will have relaxedsignificantly, thereby causing a significant reduction in thepolarization of the ¹²⁹Xe.

[0089] As a result, it becomes necessary to store the resultingpolarized ¹²⁹Xe at temperatures of 4.2K under a magnetic field of 1T toensure that the polarization remains for sufficiently long enough toallow the hyperpolarized ¹²⁹Xe to be used.

[0090] However, this problem can be overcome by the introduction of ⁴Heinto the cell 1. This has two main effects.

[0091] Firstly, the ⁴He has no nuclei spin or dipole moment andtherefore does not have relaxation enhancing properties of ³He.

[0092] Secondly, by virtue of its 33% great atomic mass and hencereduced quantum zero point motion, the ⁴He atoms will displace the ³Heatoms from the solidified layer adjacent the ¹²⁹Xe.

[0093] Accordingly, the addition of sufficient ⁴He causes displacementof the ³He such that ⁴He forms several monolayers on the surface of thepolarized ¹²⁹Xe. This is achieved by introducing the ⁴He at atemperature of below 2.2K and typically at about 300 mK so that the ⁴Heforms a superfluid film that flows freely over the ³He. As more ⁴He isadded, the ⁴He capillary condenses and covers the ¹²⁹Xe so that the ³Heis displaced.

[0094] The ³He can then be pumped out of the cell 1 using the vacuumpump. Due to the displacement of the solidified ³He, this allows the ³Heto be removed by raising the temperature of the cell 1 to about 300 mKsuch that the liquid ³He evaporates, so that it can be pumped out.

[0095] Once this has been completed, the ¹²⁹Xe can be separated from the⁴He. This can be achieved in one of two ways.

[0096] Firstly, the cell 1 can be raised in temperature to above 2.2K sothat the ⁴He evaporates and this in turn can be removed using the vacuumpump. This is possible because the ⁴He does not enhance the relaxationof the ¹²⁹Xe and does not bind as strongly to the ¹²⁹Xe as the ³He.Alternatively, the solid ¹²⁹Xe can be removed from the cell 1 therebyleaving the liquid ⁴He in the cell.

[0097] Storing the Polarized ¹²⁹Xe

[0098] Once the ¹²⁹Xe has been separated from the ³He and/or the ⁴He the¹²⁹Xe can be stored. In order to prolong the presence of thepolarization, it is necessary to store the ¹²⁹Xe at low temperatures andin the presence of a magnetic field. The lower the temperatures and thehigher the field then the longer the polarization can be maintained.

[0099] As mentioned above, if the ³He is removed by evaporation only, itis necessary to store the ¹²⁹Xe at a temperature of 4.2K in a magneticfield of 1T.

[0100] However, if ⁴He has been used to displace the ³He then the ¹²⁹Xecan be stored at a temperature of 4.2K in a magnetic field ofapproximately 100 mT in strength which can be generated by an additionalcoil arrangement (not shown) or by the high-field magnet arrangement 10.

[0101] In either case, this ensures that the ¹²⁹Xe maintains apolarization sufficient for carrying out an MRI or NMR experiment forover 1 day.

[0102] It will be realised that the ¹²⁹Xe can be stored in the cell 1,if the ⁴He has been removed. Alternatively, the ¹²⁹Xe can be extractedfrom the cell 1, and transferred to an alternative storage facility.

[0103] When it is desired to use the ¹²⁹Xe as a polarized gas in an MRIexperiment, the solidified ¹²⁹Xe can simply be heated in the presence ofa magnetic field so that it evaporates into the vapour phase. When thisoccurs, although the relaxation time of the ¹²⁹Xe will increase, theexperiment can be arranged to use the ¹²⁹Xe immediately so as to ensurethat significant polarization remains.

[0104] Thus, for example, for MRI imaging of the lungs of a patient, thesolidified ¹²⁹Xe can be evaporated and warmed to room temperature. Thiscan then be mixed with a suitable quantity of a buffer gas allowing thepatient to breathe the mixture. MRI imaging can then be carried out byplacing the patient in a magnetic field and performing the MRIprocedure, in the normal way.

[0105] Additional Features Of The Invention

[0106] The above example was described with respect to the use of ¹²⁹Xeas the sample gas being polarized. However, the present invention can beapplied to any noble gas and to other gas samples such as ²D or thelike.

[0107] The present invention can also be utilized to obtain polarized³He in two main ways.

[0108] Firstly, if ⁴He is used as a relaxation switch to remove the ³Hefrom the ¹²⁹Xe surface in the above described example, then the removed³He will typically have a level of polarization higher than that of the¹²⁹Xe. The ⁴He and ³He can therefore be removed from the cell 1 toprovide a gas mixture comprising hyperpolarized ³He, together with ⁴He.In this circumstance, the ⁴He acts as a buffer to help preventdepolarization of the ³He. Accordingly, the ³He and ⁴He can be removedfrom the cell 1 and subsequently separated to provide purehyperpolarized ³He.

[0109] Secondly, it is possible to hyperpolarize ³He using thetechniques outlined in the examples above by replacing the ¹²⁹Xe coatedwith ³He, with a layer of solidified ³He which has been formed on a coldsurface, such as the porous medium 8.

[0110] As in the example described above with respect to ¹²⁹He, thelayer of solid ³He is formed by coating the cooled porous medium 8. Whenthe solid ³He layer is formed, additional liquid ³He is usually presentin the surrounding region, as not all the ³He will be close enough toform strong Van der Waals bonds with the porous medium.

[0111] Once the solidified ³He layer has formed, because the solid ³Hehas a favourable relaxation rate, the solidified ³He can be polarized byapplying a magnetic field to the sample under suitable temperatureconditions (typically less than 2.2K).

[0112] Once the solidified ³He layer has been polarized, it is thennecessary to remove the surrounding liquid ³He which will not havepolarized so easily. It should be noted that the solid ³He follows aCurie law for magnetization whereas the liquid ³He follows a Pauli lawand accordingly, for a given field over temperature ratio, the solid ³Hepolarises far easier.

[0113] In order to remove the liquid ³He, liquid ⁴He is introduced in tothe cell 1. The liquid ⁴He displaces the liquid ³He from the solid ³Heand the resultant ³He and ⁴He mixture can then be heated and removed, inthe manners described above with respect to the ¹²⁹Xe example.

[0114] The polarized solidified ³He can then be removed from the cell,by heating the cell to allow the ³He to liquify or evaporate, in anapplied magnetic field, as was described above with respect to the¹²⁹Xe.

[0115] Accordingly, it will be appreciated by a person skilled in theart that the technique used to polarize the ¹²⁹Xe can also be used topolarize the ³He.

1. A method of hyperpolarizing a gas sample, the method comprising thesteps of: a. cryogenically forming a solidified gas structure from thesample gas, the solidified gas structure being surrounded by ³He; b.applying a magnetic field to the solidified gas structure and the ³He tothereby polarize the solidified gas structure; and c. removing the ³Heto thereby leave a solidified gas structure of hyperpolarized samplegas, characterized in that step (c) comprises the steps of: i.increasing the temperature of the solidified gas structure; ii.introducing ⁴He into the region surrounding the solidified gas structureto thereby displace the ³He; and iii. pumping the ³He and the ⁴He awayfrom the solidified gas structure.
 2. A method according to claim 1,wherein step (a) comprises the stepsof: i. forming a solidified gasstructure; and ii. introducing the ³He into the region surrounding thesolidified gas structure.
 3. A method according to claim 2, wherein step(ai) comprises the steps of: (1) cooling a substrate within a chamber;and, (2) introducing the sample gas into the chamber thereby causing thesample gas to condense onto the substrate.
 4. A method according toclaim 3, wherein the introduction of the sample gas into the chamber iscontrolled such that the sample gas condenses to form a solidified gaslayer on the substrate, the layer having a thickness of about 10monolayers.
 5. A method according to claim 3 or claim 4, wherein thesubstrate is a fumed silica.
 6. A method according to any of claims 2 to5, the solidified gas structure being porous and having a large surfacearea.
 7. A method according to claim 6, wherein step (aii) comprises thesteps of: (1) precooling the solidified gas structure; and, (2)immersing the precooled solidified gas structure in liquid ³He.
 8. Amethod according to claim 7, wherein solidified gas structure isprecooled to a temperature of below 4.2K.
 9. A method according to claim7 or claim 8, wherein the liquid ³He is precooled to a temperature ofbelow 100 mK.
 10. A method according to any of the preceding claims,wherein step (a) comprises forming a solid layer of ³He on thesolidified gas structure.
 11. A method according to any of the precedingclaims, wherein step (a) further comprises cooling the solidified gasstructure and the ³He to a temperature of less than 100 mK.
 12. A methodaccording to any of the preceding claims, wherein the magnetic fieldapplied during step (b) has a strength of between 10T and 20T.
 13. Amethod according to any of the preceding claims, wherein the gas sampleis a noble gas.
 14. A method according to claim 13, wherein the gassample comprises ¹²⁹Xe.
 15. A method according to claim 13 or claim 14,the method further comprising heating the solidified gas structure to atemperature greater than 161K to obtain a hyperpolarized noble gas. 16.A method according to any of the preceding claims, the method furthercomprising the step of storing the hyperpolarized gas in the form of thesolidified gas structure by maintaining the solidified gas structure ina magnetic field having a strength of between 50 mT and 1T at atemperature of below 10K.
 17. A method of performing an NMR experiment,the method comprising supplying a hyperpolarized gas generated accordingto any of the preceding claims to a cavity to be analysed and performingan NMR experiment.
 18. Apparatus for hyperpolarizing a gas sample, theapparatus comprising: a. cryogenic apparatus for forming a solidifiedgas structure from the sample gas, the solidified gas structure beingsurrounded by ³He; b. magnetic field generating assembly for applying amagnetic field to the solidified gas structure and the ³He to therebypolarize the ³He and the solidified gas structure; and c. a removalsystem for removing the ³He to thereby leave a solidified gas structureof hyperpolarized noble gas, characterized in that the removal system isadapted to carry out the steps of: i. increasing the temperature of thesolidified gas structure; ii. introducing ⁴He into the regionsurrounding the solidified gas structure to thereby displace the ³He;and iii. pumping the ³He and the ⁴He away from the solidified gasstructure.
 19. Apparatus according to claim 18, wherein the cryogenicapparatus comprises: a cell containing a porous substrate, the cellhaving an input for receiving the sample gas; a cooling system; and, acontroller for controlling the cooling system, the controller beingadapted to cause the cooling system to cool the cell thereby causing thesample gas to condense on the substrate so as to form a poroussolidified gas structure.
 20. Apparatus according to claim 19, whereinthe substrate is formed from a fumed silica.
 21. Apparatus according toclaim 19 or claim 20, the removal system comprising a pump coupled tothe cell, the controller operating to control the cooling system toraise the temperature of the cell so as to cause evaporation of the ³He,the pump being adapted to remove the ³He vapour from the cell. 22.Apparatus according to claim 21, wherein the removal system furthercomprises a source of ⁴He, the source being coupled to the input tosupply ⁴He into the cell, the ⁴He causing displacement of the ³He fromthe solidified gas structure.
 23. Apparatus according to any of claims19 to 22, wherein the controller is coupled to the magnetic fieldgenerating apparatus for controlling the magnetic field applied to thecell.
 24. Apparatus according to any of claims 18 to 23, wherein themagnetic field generating apparatus comprises at least onesuperconducting coil.